The skylight isn’t where to place HDRi backdrop unless as a specified cubemap, which is still only utilized as a reference for lighting. It doesn’t map the backdrop image to the inside of the skysphere by being placed in the skylight. Your chrome sphere is simply a 3D mesh, a sphere, with a partially reflected image of the scene. It is not currently set to reflect the HDRi backdrop image which is plugged into the skylight (the night sky image I see in the skylight cubemap slot). How did you get that confused with the rendering of a complete backdrop? Even the HDRi backdrop actor comes with a dynamic skylight as its component, not as a skylight with HDRi image as component. The HDRi backdrop actor is the parent of the skylight, in no uncertain yet not easily explained terms. A skylight is for lighting, a backdrop actor is for mapping an image to the skysphere (or the background). And the above is correct that having an image with no ground in it is going to require painting or somehow adding a ground…in order to get the reflection of the ground in the chrome sphere (instead of an amorphous black area that corresponds to the reflection of the non-existent ground and any other space that is not occupied by a rendered image or color).
If it wasn’t done yet, put the skylight inside the HDRI backdrop actor, unless there is one in it already. In the doc it states to not have 2+ skylights because it could ruin things or not achieve the intended result. So ensure there is ONE skylight in the scene, and that it’s a component in the HDRI backdrop actor (click + drag it on the HDRI in the world outliner list). Set it to Movable if it’s not that. Place that night-time sky image in the HDRI for its background image which gets mapped to the inside of the HDRI backdrop mesh type (dome or box, also in the HDRI settings). Then try and test the same image in the ONE skylight that’s a component of the HDRI to see how it lights the scene. A darker image produces a darker scene, while a lighter / brighter image produces a lighter scene. The more of a color range there is in the skylight cubemap image, then the further extent it may have to light a scene. In other words, a cubemap is for lighting the scene from the color/light data in the image that is set as the cubemap.